Figure 3. Different points located in the SFME system for characterization.
Furthermore, in this study, methyl orange (MO) is used as a probe for polarity measurements of the SFME system, as it is sensitive to that of the local micro-environment, giving a red shift on its maximum absorption wavelength (λmax) in a medium possessing higher polarity and vice versa.31,32 Therefore, the measurement of different microemulsions at the fixed RP/O value of 8.16 (Line Ⅰ in Figure 3) was examined by UV-vis spectroscopy (Figure S2A). The results show that the λmax of MO exhibits a red-shift (from 416.3 to 421.9 nm) with an increase of water content (WW), confirming the enhanced polarity of the microemulsion. At the same time, this linear increase in the λmax of MO with the increase of WWindicates clearly that there is no change of microemulsion type in the test area and all the given systems are assigned to O/W microemulsions, this also validates the nature of the subregions determined above. Moreover, the polarity of the microemulsion at point c in Figure 3 is compared with those of common solvents (Figure S2B). It can be found that the λmax in pure water and n -propanol are centered at 464.8 and 414.6 nm, respectively, while it peaks at 422.0 nm in the binary system without octane at a RP/W value of 4.78. However, when a microemulsion with a RP/W value of 4.78 contains 9.2 wt. % octane (point c in Figure 3), the λmax of MO shifts to 419.2 nm. Therefore, the micro-polarity of the as-constructed microemulsion system exists between those of bulk water and n -propanol.
Moreover, the formation mechanism of different microemulsions was investigated using FT-IR analysis to obtain information about their intermolecular and intramolecular interactions.33 The spectra of microemulsions with increasing WW (Line Ⅰ in Figure 3) are shown in Figure 4. There is only one strong and broad adsorption peak in the range of 3000 to 3800 cm-1 for each microemulsion system, which is attributed to the stretching vibration of hydroxyl groups. Interestingly, this adsorption peak is broadened and shifts to lower wave numbers with the increasing WW (Figure 4A) and it can be contributed to the formation of hydrogen bonds by the addition of water.34 In order to clearly demonstrate this effect, the hydroxyl band was deconvoluted using an appropriate method described by Gao et al. 35 Three distinct water species can be identified, i.e. , trapped-, bound- and free-water. According to the fitted results (Figure 4B), trapped-water (ca.3610 cm-1) exists as a comparatively small fraction in this system and is considered to be dissolved in octane or bonded with the alkyl chain of the n -propanol molecule. The bound-water (ca. 3480 cm-1) is identified as forming hydrogen bonds with the hydroxyl group of n -propanol. The absorbance observed at 3348 cm-1 is characterized as belonging to the O-H stretching vibration of n -propanol, and the one centered at 3210 cm-1 is assigned to that of free water due to the formation of strong hydrogen bounds among themselves.36